diff --git a/core/src/main/java/org/bitcoinj/utils/ExchangeRate.java b/core/src/main/java/org/bitcoinj/utils/ExchangeRate.java
index d3b8468d..71980f06 100644
--- a/core/src/main/java/org/bitcoinj/utils/ExchangeRate.java
+++ b/core/src/main/java/org/bitcoinj/utils/ExchangeRate.java
@@ -33,14 +33,16 @@ public class ExchangeRate implements Serializable {
 
     /** Construct exchange rate. This amount of coin is worth that amount of fiat. */
     public ExchangeRate(Coin coin, Fiat fiat) {
+        checkArgument(coin.isPositive());
+        checkArgument(fiat.isPositive());
+        checkArgument(fiat.currencyCode != null, "currency code required");
         this.coin = coin;
         this.fiat = fiat;
     }
 
     /** Construct exchange rate. One coin is worth this amount of fiat. */
     public ExchangeRate(Fiat fiat) {
-        this.coin = Coin.COIN;
-        this.fiat = fiat;
+        this(Coin.COIN, fiat);
     }
 
     /**
diff --git a/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java b/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java
index 8c8f2b0f..2444cf0c 100644
--- a/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java
+++ b/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java
@@ -74,4 +74,19 @@ public class ExchangeRateTest {
         ExchangeRate rate = new ExchangeRate(Fiat.parseFiat("XXX", "1000000000"));
         rate.coinToFiat(Coin.parseCoin("-1000000"));
     }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void constructMissingCurrencyCode() {
+        new ExchangeRate(Fiat.valueOf(null, 1));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void constructNegativeCoin() {
+        new ExchangeRate(Coin.valueOf(-1), Fiat.valueOf("EUR", 1));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void constructFiatCoin() {
+        new ExchangeRate(Fiat.valueOf("EUR", -1));
+    }
 }